bivalve mollusk, c. 1500 (in clam-shell), originally Scottish, apparently a particular use of Middle English clam "pincers, vice, clamp" (late 14c.), from Old English clamm "bond, fetter, grip, grasp," from Proto-Germanic *klam- "to press or squeeze together" (source also of Old High German klamma "cramp, fetter, constriction," German Klamm "a constriction"), possibly from a PIE *glem- or *glom- "contain, embrace" (see glebe).
If this is right then the original reference is to the shell. Clam-chowder attested from 1822. To be happy as a clam is from 1833, but the earliest uses do not elaborate on the notion behind it, unless it be self-containment.
"to dig for clams," 1630s, American English, from clam (n.). Clam up "be quiet" is 1916, American English, but clam was used in this sense as an interjection mid-14c.
"that part of an animal body between the head and the trunk and which connects those parts," Middle English nekke, from Old English hnecca "neck, nape, back of the neck" (a fairly rare word) from Proto-Germanic *hnekk- "the nape of the neck" (source also of Old Frisian hnekka, Middle Dutch necke, Dutch nek, Old Norse hnakkr, Old High German hnach, German Nacken "neck"), with no certain cognates outside Germanic, though Klein's sources suggest PIE *knok- "high point, ridge" (source of Old Irish cnocc, Welsh cnwch, Old Breton cnoch "hill").
The more usual Old English words were hals (the general Germanic word, cognate with Gothic, Old Norse, Danish, Swedish, Dutch, German hals), from Proto-Germanic *halsaz, which is perhaps cognate with Latin collum (see collar (n.)); and sweora, swira "neck, nape," probably also from a PIE root meaning "column" (cognate with Old English swer "column," Sanskrit svaru- "post").
Oxen and other draught animals being yoked by the neck, it became a symbol of burdens, of submission or subjugation, and also resistance or obstinacy (compare stiff-necked). Figuratively, "life" (late 15c.) from the breaking or severing of the neck in legal executions. Meaning "narrow part at the top of a bottle" is from late 14c.; meaning "part of a garment which covers the neck" is from 1520s. Meaning "long, slender part of a stringed musical instrument" is from 1610s.
Sense of "isthmus, long, narrow strip of land connecting two larger ones" is from 1550s. Phrase neck of the woods (American English) is attested from 1780 in the sense of "narrow stretch of woods;" 1839 with meaning "settlement in a wooded region." To stick (one's) neck out "take a risk" is recorded by 1919, American English. Horses running neck and neck "at an equal pace" is attested from 1799; to win by a neck is from 1823. To be up to the neck "have a lot of" at first (mid-19c.) suggested "fed full," but since c. 1900 it has implied "in deep."
"to kiss, embrace, caress," 1825 (implied in necking) in northern England dialect, from neck (n.). Compare Middle English halsen "to embrace or caress affectionately, to fondle sexually," from hals (n.) "neck." Earlier, neck as a verb meant "to kill by a strike on the neck" (mid-15c.). Related: Necked.
Old English lang "having a great linear extent, that extends considerably from end to end; tall; lasting," from Proto-Germanic *langa- (source also of Old Frisian and Old Saxon lang, Old High German and German lang, Old Norse langr, Middle Dutch lanc, Dutch lang, Gothic laggs "long").
The Germanic words perhaps are from PIE *dlonghos- (source also of Latin longus "long, extended; further; of long duration; distant, remote," Old Persian darga-, Persian dirang, Sanskrit dirghah "long"), from root *del- (1) "long" (source also of Greek dolikhos "long," endelekhes "perpetual"). Latin longus (source of prolong, elongate, longitude, etc.) thus is probably cognate with, but not the source of, the Germanic words. The word illustrates the Old English tendency for short "a" to become short "o" before -n- (also retained in bond/band and West Midlands dialectal lond from land and hond from hand).
Also in Old English in reference to time, "drawn out in duration," with overtones of "serious." The old sense of "tall" now appears to be dialectal only, or obsolete. For long "during a long time" is from c. 1300. To be long on something, "have a lot" of it, is from 1900, American English slang. A long vowel (c. 1000) originally was pronounced for an extended time. Mathematical long division is from 1808. Sporting long ball is from 1744, originally in cricket. Long jump as a sporting event is attested from 1864. A long face, one drawn downward in expression of sadness or solemnity, is from 1786. Long in the tooth (1841 of persons) is from horses showing age by recession of gums (but not in this sense until 1870). Long knives, name Native Americans gave to white settlers (originally in Virginia/Kentucky) is from 1774, perhaps a reference to their swords. Long time no see, supposedly imitative of American Indian speech, is first recorded 1919 as Chinese English.
adverbial suffix indicating direction, from Old Norse -langr, from langr "long" (adj.); see long (adj.). Displaced native -ling.
Old English lange, longe "for a length of time, a long time; far, to a great extent in space," from long (adj.). Old English also had langlice (adv.) "for a long time, long, at length." Longly (adv.) is rarely used. No longer "not as formerly" is from c. 1300; to be not long for this world "soon to die" is from 1714.
Middle English longen, from Old English langian "to yearn after, grieve for," literally "to grow long, lengthen," from Proto-Germanic *langojan, which probably is connected with the root of long (adj.). Cognate with Old Norse langa, Old Saxon langon, Middle Dutch langhen, Old High German langen "to long," German verlangen "to desire." Related: Longed; longing.